Development of A ThreeDimensional MODFLOW
Flow Model For Yucca Mountain, Nevada 

Top: Oblique view of 30layer MODFLOW
flow model covering a 28 km x 43 km area near Yucca Mountain, NV;
colors indicate different hydrogeologic features.
Below: Satellite image showing calibrated hydraulic heads, and flow
paths and a well capture zone delineated using the MODPATH particle
tracking code. 



Southwest Research Institute 


Sponsor: U.S. Nuclear Regulatory Commission 
Principal Investigators:
James Winterle 

Program Brief
Statement of Problem: The client
needed a saturated zone flow model of the Yucca Mountain region that
could be used to evaluate the effects on flow paths of various
alternative conceptual models of hydrogeologic structure and hydrologic
boundary conditions.
Approach and Accomplishments:
UA threedimensional groundwater flow model was constructed for the area
surrounding Yucca Mountain using the MODFLOW code. The computational
grid for this model covers a 28.5 × 41.4km area that extends vertically
from 1,200 m above sea level to 1,500 m below sea level. The model
contains 30 horizontal grid layers, varying in thickness from 50 m to
200 m, with the thinnest grid layers assigned at and below the water
table where flow paths from Yucca Mountain might occur. Each of the 30
layers is uniformly divided into 300m horizontal squares for a total of
393,300 computational cells. An algorithm was developed to assign
hydrologic properties to each model grid cell based on correspondence to
hydrogeologic units and fault zones identified in an Earth Vision
geographic information system database for the region. Several
calibrated versions of the model, representing alternative sets of
boundary conditions, analytical solutions (e.g., confined vs.
unconfined), recharge rates, and fault zone properties, were developed
to evaluate the effects of model uncertainties on projected flow paths
from the proposed nuclear waste repository at Yucca Mountain. Flow paths
were evaluated using the MODPATH particletracking algorithm, a
companion code to MODFLOW.
Client Benefits: The results of
these modeling analyses are used to develop simplified abstractions of
groundwater flow paths and velocities that are incorporated into
riskbased totalsystem performance assessments to evaluate relative
importance of various model and parameter uncertainties. Such analyses
permit focusing of research activities on those uncertainties that are
shown to be most important. Other uses of the model include delineation
of well capture zones and studying the potential effects of future
climate changes on groundwater flow paths and velocities. 